Redraw only dirty area for accelerated 2D Canvas.

Source/core/html/HTMLCanvasElement.h

 /*
  * Copyright (C) 2004, 2006, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 136 matching lines @@
     // CanvasImageSource implementation
     virtual PassRefPtr<Image> getSourceImageForCanvas(SourceImageMode, SourceImageStatus*) const OVERRIDE;
     virtual bool wouldTaintOrigin(SecurityOrigin*) const OVERRIDE;
     virtual FloatSize sourceSize() const OVERRIDE;
 
     // ImageBufferClient implementation
     virtual void notifySurfaceInvalid() OVERRIDE;
 
     virtual void trace(Visitor*) OVERRIDE;
 
+    FloatRect dirtyRect() { return m_dirtyRect; }
+
 protected:
     virtual void didMoveToNewDocument(Document& oldDocument) OVERRIDE;
 
 private:
     explicit HTMLCanvasElement(Document&);
 
     virtual void parseAttribute(const QualifiedName&, const AtomicString&) OVERRIDE;
     virtual RenderObject* createRenderer(RenderStyle*) OVERRIDE;
     virtual bool areAuthorShadowsAllowed() const OVERRIDE { return false; }
 
@@ skipping 33 matching lines @@
     OwnPtr<ImageBuffer> m_imageBuffer;
     mutable OwnPtr<GraphicsContextStateSaver> m_contextStateSaver;
 
     mutable RefPtr<Image> m_presentedImage;
     mutable RefPtr<Image> m_copiedImage; // FIXME: This is temporary for platforms that have to copy the image buffer to render (and for CSSCanvasValue).
 };
 
 } //namespace
 
 #endif